This invention relates to methods for electrically connecting a flip chip to a substrate.
Flip chip mounting is an increasingly popular technique for directly electrically connecting an integrated circuit chip to a substrate such as a circuit board. In this configuration, the active face of the chip is mounted face down, or xe2x80x9cflippedxe2x80x9d on the substrate. The electrical bond pads on the flip chip are aligned with corresponding electrical bond pads on the substrate, with the chip and substrate bond pads electrically connected by way of an electrically conductive material. The flip chip mounting technique eliminates the use of bond wires between a chip or chip package and the substrate, resulting in increased reliability of the chip-to-substrate bond.
A wide range of electrically conducting compositions have been proposed for making the interconnection between flip chip and substrate bond pads.
Solder balls, gold bumps, gold stud bumps, and other conventional metal bump configurations have been used extensively. Aside from metallic compositions, electrically conducting polymer compositions are gaining wide acceptance as flip chip interconnection bump materials. In a flip chip mounting technique employing polymer interconnections, electrically conductive polymer bumps are formed on the bond pads, typically of the flip chip, and are polymerized or dried to effect bonding to the substrate bond pads, whereby both an electrical and a mechanical adhesive bond between the flip chip and the substrate bond pads is produced. Electrically conductive polymer materials are particularly well-suited for flip chip mounting techniques because of their ease of application, because they eliminate many of the unwanted characteristics of metallic interconnections, e.g., solder flux, and because for some polymer materials reworkability of faulty flip chips is enabled by simple heating of the material.
Conventionally, once a flip chip is bonded to a substrate, whether by metallic or by polymer bump interconnections between the chip and substrate bond pads, an underfill material is dispensed between the chip and the substrate. The underfill material is typically provided as a liquid adhesive resin that can be dried or polymerized. The underfill material provides enhanced mechanical adhesion and mechanical and thermal stability between the flip chip and the substrate, and inhibits environmental attack of chip and substrate surfaces.
The invention provides processes that exploit the superior bonding capabilities of electrically conductive polymer materials for bonding a flip chip to a substrate in a manner that maximizes reliability of the bonding operation. In a first mounting process provided by the invention, electrically conductive polymer bumps are formed on bond pads of a flip chip and the flip chip polymer bumps are at least partially hardened. Electrically conductive polymer bumps are formed on bond pads of a substrate, and a layer of electrically insulating adhesive paste is then applied on the substrate, covering the substrate polymer bumps with the adhesive.
The bond pads of the flip chip are then aligned with the bond pads of the substrate and the at least partially hardened flip chip polymer bumps are then pushed through the substrate adhesive and at least partially into the substrate polymer bumps. Preferably, the flip chip polymer bumps are pushed through the substrate adhesive and into the substrate polymer bumps to a depth sufficient to produce electrical connections between the flip chip polymer bumps and the substrate bond pads. For some applications, it can be preferred to push the flip chip polymer bumps through the adhesive and through the substrate polymer bumps to directly contact the flip chip polymer bumps with the substrate bond pads.
This process results in electrical and mechanical bonding of the polymer bumps between the chip and substrate bond pads, either directly or via the substrate polymer bumps, even though the adhesive layer was applied on the substrate in a manner that covered the substrate polymer bumps. The flip chip polymer bumps displace the adhesive and penetrate the substrate polymer bumps by the mounting technique of the invention. As a result, the area around the polymer bumps between the chip and the substrate is filled with the adhesive, in the manner of an underfill. A separate, post-bond underfill process is therefore not required.
In addition, the substrate polymer bumps can operate as an intermediary, compensating for non-coplanar flip chip bumps. If some flip chip bumps are shorter than others, it can be assured that most, if not all, flip chip bumps are electrically connected to substrate bond pads, if not directly, then by way of the substrate polymer bumps. The overall reliability of the bonding process is thereby significantly enhanced by the inclusion of substrate polymer bumps.
In embodiments provided by the invention, the adhesive paste applied to the substrate can be at least partially dried or at least partially cured, as appropriate for the selected paste material, before the step of pushing the polymer bumps through the adhesive on the substrate. Similarly, the step of at least partially hardening the polymer bumps can be carried out by at least partially drying or by fully polymerizing the polymer bumps, as appropriate for the selected bump material.
In further embodiments provided by the invention, heat is applied to the flip chip as the bumps are pushed through the adhesive on the substrate. Heat can also be applied to the flip chip after the polymer bumps contact the bond pads of the substrate. Pressure is preferably applied to the flip chip during the bonding process for a selected duration, based on material characteristics of the adhesive and of the polymer bumps.
The height of the substrate polymer bumps, as-formed, preferably is between about 30% and about 150% of the height of the flip chip polymer bumps as-formed. The diameter of the substrate polymer bumps, as-formed, preferably is between about 10% and about 70% greater than the diameter of the flip chip polymer bumps as-formed. Preferably, the flip chip polymer bumps have a bump height as-formed that is greater than the adhesive paste thickness as-applied on the substrate, more preferably having a bump height that is at least about 25% greater than the adhesive paste thickness; the as-applied adhesive thickness is preferably at least as great as the substrate polymer bump height as-formed.
The flip chip polymer bumps, the substrate polymer bumps, and the adhesive paste can each be distinctly formed of, e.g., a thermoplastic material, a thermoset material, or a B-stage thermoset material. The polymer bumps can include hard particles, preferably that have jagged edges or sharp points which protrude from the bump. Such particles are preferably electrically conductive.
Both the flip chip and substrate polymer bump formation and the adhesive application to the substrate can be carried out by, e.g., a stenciling process or a screen printing process. The adhesive can further be applied by, e.g., a dispensing process. Preferably, the adhesive paste is stenciled with a stencil that includes openings at stencil locations corresponding to substrate polymer bump locations.
The invention provides a further method for mounting a flip chip on a substrate. In this method, electrically conductive polymer bumps are formed on bond pads of a flip chip and the flip chip polymer bumps are at least partially hardened. A layer of electrically insulating adhesive paste is formed on a substrate having bond pads, covering the bond pads with the adhesive. The bond pads of the flip chip are aligned with the bond pads of the substrate, and then the at least partially hardened flip chip polymer bumps are pushed through the substrate adhesive with pressure sufficient for the flip chip polymer bumps to directly contact and to deform the bond pads of the substrate.
It is recognized that such bond pad deformation can significantly enhance the electrical connection between a polymer bump and a bond pad. This is understood to be enabled by a range of mechanisms including, among other things, enhancement of mechanical contact area between the polymer bump and bond pad. It is further recognized that the structural integrity and reliability of the bond between a polymer bump and a bond pad is significantly enhanced by such bond pad deformation. The invention provides a discovery that flip chip bumps formed of a polymer material have sufficient mechanical integrity to deform substrate bond pads.
Preferably, heat and pressure are applied to the flip chip as the flip chip polymer bumps are pushed through the substrate adhesive. The temperature of the chip heating and the degree of applied chip pressure are preferably selected based on the thickness of the substrate bond pads to enable deformation of the substrate bond pads. Bond pad deformation of less than about 50% of flip chip polymer bump height as-formed is preferred.
In accordance with the invention, the flip chip polymer bumps can be pushed through the substrate adhesive with a pressure sufficient for the flip chip polymer bumps to be burnished by the substrate bond pads. It can be preferred that the pressure be sufficient for the flip chip polymer bumps to be vertically compressed between the flip chip and the substrate to a compressed height that is less than the as-formed flip chip polymer bump height.
In embodiments provided by the invention, the substrate can be mechanically flexible, whereby the flip chip polymer bumps deform the substrate as well as the substrate bond pads. The substrate bond pads can be provided as copper, optionally including nickel and gold layers, preferably with each layer being sufficiently thin to accommodate bond pad deformation.
The flip chip mounting techniques of the invention are widely applicable to a range of substrate materials and flip chip mounting configurations. The flexibility in adhesive application and polymer bump formation methods allow for versatility in material formulation for application-specific considerations. In general, the flip chip mounting technique can be employed as a superior alternative for most conventional flip chip mounting processes that employ solder or other metallic bumps and conventional post-bond underfill processes, resulting in enhanced mounting quality and improved process efficiency.
Other features and advantages of the flip chip mounting methods of the invention will be apparent from the following description and accompanying drawing, and from the claims.